This invention relates to improved controlled permeability films and to improved methods of making such films.
Controlled permeability films are particularly concerned with allowing gases such as oxygen and carbon dioxide to pass through the film. These films find application in the storage of products which are affected by the presence of gases such as ethylene, oxygen and carbon dioxide and which respire and produce or consume these gases. Conditions for the optimal storage of horticultural products are influenced by factors which include crop species, cultivar, maturity, growing conditions, quality, relative humidity, temperature, packaging and storage duration. Control of carbon dioxide and/or oxygen concentration about such products has been shown to increase the product""s storage life. Storage under controlled atmosphere involves modifying the concentration of oxygen, carbon dioxide, water vapour, ethylene and other gases. Rates of respiration vary, with broccoli having a high rate and lettuce having a relatively low rate. Controlled atmosphere research with broccoli has shown that oxygen levels below approximately 1% and CO2 levels higher than about 15% independently induced offensive off-odors and off-flavors. Optimum oxygen and carbon dioxide concentrations reported for broccoli range from approximately 1 to 2.5% and about 5 to 10% respectively. Product life is extended using controlled atmosphere because respiration is slow and pathogen growth is inhibited.
In modified atmosphere packaging, the respiration behavior of the produce is utilized to control the composition of the atmosphere about the produce. The film permeability is matched to the expected respiration behavior as influenced by temperature and atmosphere conditions to provide an optimum carbon dioxide and oxygen content in the package. The accumulated oxygen and carbon dioxide concentrations in such a package will be related to the rate at which they are consumed or generated by the produce and the package permeability, by a simple mass balance. The sensitivity of this balance to oxygen and carbon dioxide permeability requires consistent manufacturing processes to make controlled permeability films.
Perforated films can produce controlled permeability but have the disadvantage of not acting as a barrier to microorganisms and/or moisture.
Controlled permeability can be achieved by careful selection of the polymers used and patent specification WO94/25271 is one example where two different polymers are used in separate layers. This patent also recognizes that clarity, printability, and feel of the film are also critical factors for packaging film used at the retail level. One disadvantage is that the range of permeability is limited. Another means of achieving controlled permeability is disclosed in patent specifications WO92/02580 and WO95/07949. Permeability is achieved by incorporating into the film filler particles larger than the intrinsic thickness of the film. The filler particles are inert and porous. The permeability can be varied by treating the filled film with heat and/or pressure or by stretching the film. The object of such a method is that one film can be treated to provide a range of films with differing permeability. This would enable a food packer to purchase one type of film and treat it on site to produce an appropriate film for each product to be packed. One shortcoming with this type of film is that it is not suitable for retail packaging because the clarity and feel of the film is not acceptable to the majority of consumers.
Another use of controlled permeability films is the storage of nematodes. As disclosed in Patent specification PCT/AU92/00041, the entire disclosure of which is incorporated herein by reference, nematodes may be successfully packed for storage and transport using controlled permeability films.
It is an object of this invention to overcome the shortcomings of the prior art and provide a consumer acceptable film able to provide a wide range of permeability.